1. Field of the Invention
The present invention relates to an outboard motor used to propel a boat and capable of being detachably clamped to the transom of the boat and, more particularly, to a gasket for such an outboard motor.
2. Description of the Related Art
An outboard motor includes a case assembly held by a bracket on the hull of a boat, an engine cover detachably attached to the upper end of the case assembly, and an engine disposed in an engine room defined by the case assembly and the engine cover. A vertical member having a cross section defined by a substantially wing-shaped outline is extended vertically downward from a part, corresponding to the engine room, of the case assembly. A vertical drive shaft connected to the output shaft of the engine is extended in the vertical member. A horizontal propeller shaft parallel to a propelling direction is interlocked with a lower part of the drive shaft. A propeller is mounted on the propeller shaft.
The engine of the outboard motor is a vertical-axis engine having a vertical crankshaft. A multicylinder engine for an outboard motor has cylinders placed in vertical arrangement with their axes extended in a horizontal direction parallel to a propelling direction.
An engine has a cylinder block provided with a cylinder or cylinders, and a cylinder head joined to the cylinder block so as to define a combustion chamber or combustion chambers. Generally, low-power outboard motors, for propelling a boat, having a small power capacity on the order of 2 PS, are provided with an air-cooled engine and those having a large power capacity are provided with a water-cooled engine. The outboard motor provided with a water-cooled engine pumps up water through a submerged part thereof by a cooling water pump and sends the water through a water passage toward the water-cooled engine. The water flows through a water jacket formed in the cylinder block and the cylinder head so as to surround the cylinders forming combustion chambers to cool the cylinder block and the cylinder head, flows down into a lower part of the outboard motor and is discharged from the outboard motor.
When the engine is a four-stroke engine, the engine is provided with an oil pan for holding lubricating oil for lubricating parts of the engine.
A gasket is held between the cylinder block and the cylinder head to make a pressure-tight joint for preventing the leakage of the combustion gas. Gaskets are held also between the cylinder block and a mount case included in the case assembly and between the mount case and the oil pan, respectively, to prevent the leakage of the lubricating oil. Those gaskets are provided with openings corresponding to openings of the cooling water passage and the edges of those gaskets are exposed to the mist of cooling water containing the exhaust gas.
Generally, the gaskets are formed of a stainless steel, while engine parts including the cylinder block, the cylinder head and the oil pan are formed of aluminum alloys to form the engine in a lightweight structure. Consequently, the gaskets and the engine parts are corroded by electrochemical corrosion due to potential difference between each gasket and an engine part continuous with the gasket.
An aluminum-core cylinder head gasket is disclosed in, for example, JP 6-11042 A as a corrosion preventing means for preventing the electrochemical corrosion of the engine parts of the outboard motor. This known cylinder head gasket has a core formed by working a metal sheet having an electrolytic potential approximately equal to that of the surfaces of first and second engine parts of a nonferrous metal of a water-cooled engine, and having upper and lower surfaces, and a coating having first and second layers respectively coating the upper and the lower surface of the core.
More concretely, the cylinder head and the cylinder block are formed by casting aluminum and are joined together with bolts. The gasket is held between the cylinder head and the cylinder block. The gasket has an aluminum core, first and second coating layers on the aluminum core, made of heat-resistant aramid fibers or a synthetic elastomer capable of making a water-tight joint, and a sealing member of a stainless steel for providing a seal around the combustion chamber. The gasket is provided with openings corresponding to openings of the cooling water passage. The aluminum core is sandwiched between the first and second coating layers to make the potential of the gasket equal to that of the engine part, such as the cylinder block.
The known gasket is complicated in construction, is difficult to manufacture and hence is expensive. Moreover, the aluminum core of the gasket is subject to electrochemical corrosion.
The present invention has been made to solve those problems and it is therefore an object of the present invention to provide a gasket including a gasket base formed of a ferrous metal and coating members formed of a nonferrous metal and respectively coating surfaces of the gasket base, resistant to electrochemical corrosion and capable of ensuring reliable sealing and to provide a water-cooled engine provided with the foregoing gasket.